Fire boats battle a fire at the off shore oil rig Deepwater Horizon April 21, 2010 in the Gulf of Mexico off the coast of Louisiana. Multiple Coast Guard helicopters, planes and cutters responded to rescue the Deepwater Horizons 126 person crew after an explosion and fire caused the crew to evacuate. (Photo by U.S. Coast Guard via Getty Images)

Photo: U.S. Coast Guard, Handout

Fire boats battle a fire at the off shore oil rig Deepwater Horizon...

The decision by U.S. District Judge Lee Rosenthal also affirmed the Chemical Safety Board's jurisdiction to investigate the accident, at least temporarily ending a long-running dispute about the independent agency's power.

At issue were five subpoenas that the board issued to Transocean in 2010 and 2011 as the agency sought to learn more about the explosion of the company's Deepwater Horizon drilling rig in the Gulf of Mexico. The Justice Department argued on behalf of the board's authority and subpoenas.

Transocean contended that the board did not have authority to probe offshore accidents and oil spills, and said it already had provided most of the requested documents to the Interior Department and Coast Guard as part of separate inquiries.

The federal law that created the board said it isn't authorized to investigate marine oil spills. But the agency maintained it was investigating the root causes of the deadly explosion on the rig on April 20, 2010 - not the related oil spill from BP's Macondo well.

The board further argued that the statutory prohibition applies only to investigations of transportation-related spills, while its inquiry focuses on what happened on the rig itself. The rig was tethered by a pipe to the well in the seabed, effectively making it a stationary installation, board officials said.

Rosenthal acknow-ledged a legal precedent for treating offshore drilling units as vessels, but ultimately agreed with the Chemical Safety Board's analysis because Transocean's rig was 50 miles off Louisiana, outside the U.S. waters where the National Transportation Safety Board typically is in charge of investigating vessel-related accidents.